When there is a large number of new or modified files,
"display_all_files" takes a long time, and git-gui appears to hang.

This change limits the number of files that are displayed.  This
limit can be set as gui.maxfilesdisplayed, and is 5000 by default.

A warning is shown the first time the list of files is truncated
in this GUI session.  Subsequent truncations are not mentioned to
the user.
Signed-off-by: NDan Zwell <dzwell@zwell.net>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>